1. Field of the Invention
The present invention relates to a method for cleaning a thermal printing head in use for perforating a stencil sheet and the like.
2. Background of the Invention
Stencil printing is effected by a perforating step in which perforations corresponding to an image to be printed, namely an "original", are made in a stencil sheet, and a printing step in which ink is transferred to printing paper through the perforations of the perforated stencil sheet. As stencil sheets, have widely been used heat-sensitive stencil sheets which are composed of an ink-impermeable thermoplastic film such as of polyester laminated to an ink-permeable porous substrate such as of Japanese paper.
Such heat-sensitive stencil sheets can be perforated by means of a thermal printing head which has in a longitudinal direction thereof fine heating elements arranged in a row. In other words, perforations corresponding to an original can be made in the stencil sheet by bringing the stencil sheet into contact with the thermal printing head, and heating a plurality of heating elements selected in accordance with image data of the original so as to melt the thermoplastic film of the stencil sheet.
The thermal printing head generally has an elongated configuration. The stencil sheet is perforated while it is pressed to and conveyed on the thermal printing head by means of a platen roller disposed opposite to the thermal printing head. Herein, the longitudinal direction of the thermal printing head is referred to as "horizontal scanning direction", and the direction which is transverse to the horizontal scanning direction and in which the stencil sheet is conveyed is referred to as "vertical scanning direction".
The stencil sheet is often provided on the surface of the thermoplastic film with a releasing agent layer made of fluorinated resins or the like in order to prevent scum of the molten film from sticking to heating elements upon perforation. However, when the thermal printing head is repeatedly used for perforation of stencil sheets, not only scum of the molten film but also adhesives bonding the film to a substrate such as paper and something like that gradually stick to and deposit in the neighborhood of heating elements of the printing head. Such deposits are found remarkable at the end of heating elements on the side downstream to the direction in which stencil sheets are conveyed. If deposits grow too large, close contact of the film with heating elements is interrupted upon perforation. Also, conveyance of the film is interrupted due to increase in friction between the film and the printing head, so that the film becomes difficult to be perforated, causing printed image to gradually deteriorate.
It has been a conventional operation that when deposits are found in the vicinity of heating elements of the thermal printing head, operators wipe off the deposits by use of alcohol or the like. Since it is quite troublesome to remove deposits every time they occur, a method for removing deposits, in which a polishing layer formed on part of a roll of stencil sheet is pressed to the thermal printing head, is disclosed in Japanese Patent Laid-open (Kokai) Nos. 239047/94 and 309002/95.
In such a method for removing deposits using polishing layers, life of the thermal printing head might be shortened by polishing action. Further, an additional manufacturing step is required to form the polishing layer on stencil sheets, making the manufacturing process complicated.
Furthermore, since deposits are removed by the stencil sheet only at portions where polishing layers are previously formed, interval between cleanings of the thermal printing head cannot be varied depending upon degree of deposition. When a roll of stencil sheet is provided with the polishing layer at regular intervals of a length required for making one master, that is the length of a stencil sheet required for each printing, perforation might be made in the stencil sheet at portions where the polishing layer is formed, due to expansion, shrinkage or the like. When the polishing layer is formed at the beginning and/or last end of a roll of stencil sheet, the thermal printing head is cleaned only once or twice until the roll is all spent.
The object of the present invention is to provide a method for cleaning a thermal printing head, which does not require a special stencil sheet nor damage the thermal printing head, but makes it possible to remove deposits as required.